The date for which a prime minister begins his or her term has been determined by the date that he or she is sworn into his or her portfolio, as an oath of office as prime minister is not required.[3] However, since 1957, the incoming prime minister has sworn an oath as prime minister.[3] Before 1920, prime ministers' resignations were accepted immediately by the governor general and the last day of the ministries were the date he died or the date of resignation.[3] Since 1920, the outgoing prime minister has only formally resigned when the new government is ready to be formed.[3] The Interpretation Act of 1967 states that "where an appointment is made effective or terminates on a specified day, that appointment is considered to be effective or to terminate after the end of the previous day".[3] Thus, although the outgoing prime minister formally resigns only hours before the incoming ministry swears their oaths, both during the day, the ministries are effectively changed at midnight the night before. Some sources, including the Parliament of Canada, apply this convention as far back as 1917.[4] Two prime ministers have died in office: John A. Macdonald (1867–1873, 1878–1891), and John Thompson (1892–1894). All others have resigned, either after losing an election or upon retirement.

Minister without Portfolio; Succeeded on Macdonald's death due to objections to the Catholic John Thompson. In ill health; retired. First prime minister born in what would become Canada, and first of only two prime ministers to serve while in the Senate.

* The Interpretation Act of 1967 states that "where an appointment is made effective or terminates on a specified day, that appointment is considered to be effective or to terminate after the end of the previous day." Under the Act, Prime Ministers' tenures are therefore credited as having concluded at the end of their last full day in office (the earlier date given), although their resignation was received by the Governor General on the following day. This provision applies to P. Trudeau in 1979[45] and 1984,[46] Clark,[47] Turner,[48] Mulroney,[49] Campbell,[50] Chrétien,[51] Martin,[51] and Harper.[51]

As of November 2018, there are seven living former prime ministers of Canada, the oldest being John Turner (born 1929). The most recent former Prime Minister to die was Pierre Trudeau(1968–1979, 1980–1984), on September 28, 2000. The living former prime ministers, in order of service, are: